The present invention relates to laser radars and more particularly to a laser radar employing a pulse-tone waveform for determining both range and velocity of a target.
Laser radars operate by launching a beam of laser light toward a target and receiving the return signal. Range can be determined by the time of flight of a pulse from the laser until detection of the return signal by a receiver. Range accuracy depends on the narrowness of the pulse so that a pulse having a very short duration will result in the highest range accuracy. On the other hand, the determination of target velocity is most readily accomplished with a laser generating a CW output. Such an output may be thought of as a tone signal because of its substantially constant amplitude. The frequency shift of such a tone is directly related to the target velocity. The utility of a waveform including both a pulse and a tone portion has been described in U.S. Pat. No. 4,298,280. Such a prior art waveform is illustrated in FIG. 1. The time of flight of the pulse portion of the pulse-tone waveform is used to determine range, and the frequency shift in the tone portion of the pulse-tone waveform is an indication of target velocity.
Though useful, the prior art pulse-tone waveform is difficult, if not impossible, to generate with a single laser. One reason for the difficulty is that a laser must be turned off to permit sufficient energy storage to generate a narrow or Q-switched pulse. During this time, the CW or tone portion may not be generated.
It is therefore an object of this invention to provide laser radar apparatus which generates and utilizes a pulse-tone waveform.
A further object of the invention is such laser radar which does not require a separate local oscillator laser.
Yet another object of the invention is laser radar apparatus capable of use in either a pulsed, CW or pulse-tone mode.